The Marcellus Shale is a sedimentary rock formation deposited over 350 million years ago in a shallow inland sea located in the eastern United States where the present-day Appalachian Mountains now stand. This shale contains significant quantities of natural gas. New developments in drilling technology, along with higher wellhead prices, have made the Marcellus Shale an important natural gas resource.
In the new hydrofracturing process, high pressure water is forced into the well during the drilling process to break up the shale deposit and release natural gas. This process typically requires from two (2) to seven (7) million gallons (MG) of water to frac a well (drill using high pressure water), and additional 0.1 to one (1) million gallons of water needed for drilling fluids to maintain down-hole hydrostatic pressure, cool the drill head and enable removal of drill cuttings. Approximately twenty to twenty-five percent (20 to 25%) of this water, an average of 1.2 MG, returns to the surface with the natural gas. The return water is heavily contaminated with metals, for example, Barium (Ba), Strontium (Sr), Magnesium (Mg), Calcium (Ca), and Iron (Fe), and in some cases radionuclides. This return wastewater is generated in two (2) stages: i) the first stage is flowback water that returns to the surface with natural gas in one (1) to three (3) weeks after the gas production starts; and ii) the second stage is produced water which continues to flow approximately three (3) weeks after gas production starts, and continues over the life of the well, approximately six (6) years. Usually, approximately twenty-one percent (21%), used in the drilling process returns to the surface as flowback water, approximately 1.06 MG. This frac (frac and produced) water, in addition to the frac fluids added by the gas drilling companies, contain a variety of contaminants such as total dissolved solids, heavy metals, organics and possibly radionuclides. This combination of contaminants can make frac water difficult and expensive to treat. Most of the total dissolved solids (TDS) in flowback water and produced water is due to sodium chloride, and on average, is three (3) to five (5) times saltier than sea water. TABLE A shows the constituents and concentration ranges usually found in untreated samples of frac water.
TABLE AAnalyte Concentration RangesSr. No.Analyte(mg/L)1.pH3.5-6.52.Iron (Fe) 10-1503.Barium (Ba)  25-4,0004.Sulfate (SO4) 5-4005.Chloride (Cl) 10,000-150,0006.Sodium (Na)10,000-50,0007.Strontium (Sr)  100-3,0008.Total Dissolved Solids (TDS) 15,000-250,0009.Calcium (Ca)  500-20,00010.Magnesium (Mg)  100-3,00011.Total Suspended Solids (TSS)  100-1,500
The above-mentioned frac water must be treated and disposed of properly. Some of the current disposal practices for frac water include: (1) storing the frac water in large ponds and tanks and reusing after being blended with clean water; (2) loading the frac water into trucks/trailers and sending/transporting it to external centralized treatment facilities for disposal or disposing in underground deep-well injection wells; and (3) treating the frac water on-site by means of known on-site treatment technologies such as Evaporation, Crystallization, Distillation, and/or multiple selective precipitation steps of five (5) or more stages, etc. Traditional treatment techniques such as evaporation and selective precipitation appear not to be economically feasible options for treating Marcellus shale wastewater.
The various wastewater treatment methods for treating Marcellus shale frac water is generally costly with higher energy consumptions, and the effectiveness and efficiency at removing the contaminants is not very good. Further, because the amount of gas wells in the Marcellus shale area are increasing at a substantial rate, there is a need in the industry to find an effective and cost efficient way to treat frac water generated from gas production.
It is advantageous to provide a cost-efficient, portable and effective process that treats frac water streams containing waste metal elements and/or compounds using a by-product such as BOF sludge generated during steel production. The advantages of this wastewater treatment process include, but not limited to, significant metals reduction, recovery and re-use of regenerated BOF sludge as a catalyst, breakdown of organic compounds including recalcitrant organic compounds, and the use of readily available equipment components that do not require specialty materials and fabrication techniques.
The gas companies vary as to the water quality suitable for drilling purposes. A suitable frac water treatment process does not necessarily have to meet “the water quality suitable for drilling,” if after adding fresh water the final water quality meets the drilling standards for the client. Generally, treated water available for drilling is approximately ⅓rd of the total volume required. Therefore, any final treated water quality can be three (3) times higher than the acceptable limits and be accepted for recycling, since dilution will bring the water quality into the acceptable range.
The following is an example of a water quality acceptable for recycling after treatment by a Major Gas Producer.
TABLE BSr. No.AnalyteUnitsAnalyte Concentration1.pH—6.0-8.52.Iron (Fe)mg/L0.3-4.03.Barium (Ba2+)mg/L163-2004.Strontium (Sr2+)mg/L323-3775.Calcium (Ca2+)mg/L226-3506.Magnesium (Mg2+)mg/L595-7007.Sodium (Na+)mg/L35,050-50,0008.Potassium (K+)mg/L  966-1,5299.Chloride (Cl−)mg/L55,400-65,00010.Sulfate (SO42−)mg/L 0-20011.Hardnessmg/L3,003-3,260as CaCO312.Total Suspended Solids (TSS)mg/L138-245